The present invention relates to a run length limited (RLL) coding method used for digital data magnetic recording. More particularly, it relates to an RLL code generation method providing higher recording density than a conventional rate 8/9(0,4/4) coding method and decoding method therefor.
Currently, research and development for effectively utilizing massive amounts of information have been conducted as the amount of data increases, particularly, in the field related to data storage devices. The end of the effort in the data storing field has been concentrated in the rapid transfer of massive amounts of information with high recording density and high reliability. To this end, a method for improving the physical properties of a data storage disk or improving the precision of an storage device is considered. Also, as an aspect of signal processing, there are provided a method for increasing the recording density of the storage device through effective coding, a method for reducing a data detection error using a signal processing technology, etc.
Generally, data to be written in a data storage device is encoded into a RLL code. RLL code is a code that limits continuous-repetition of a specific bit pattern for timing control of a sampling clock and proper signal detection, that is, the number of successive "0"s between "1" and "1" is limited to the minimum d and the maximum k.
As recently used among coding methods using the RLL code there are rate 1/2(2,7) modulation code, rate 2/3(1,7) modulation code, rate 8/9(0,3) modulation code, rate 8/9(0,4/4) modulation code, etc.
According to the rate 1/2(2,7) modulation code and rate 2/3(1,7) modulation code, "d" is equal to 1 and 2, respectively. Accordingly, interference between signals is decreased while redundancy is high due to the low code rate. Also, since the value of "k" is comparatively greater than those of the rate 8/9(0,3) and rate 8/9(0,4/4) modulation codes, it has less timing information which is helpful in the operation of a phase locked loop (PLL).
The rate 8/9(0,3) coding and rate 8/9(0,4/4) coding methods provide high recording density due to less redundancy thereof, and include much timing information due to the small "k" thereof. However, interference between signals increases since "d" is equal to "0".
Partial response maximum likelihood (PRML) pre-codes input signal to provide controlled inter-symbol interference (ISI) between the current data and the previous data, and then modifies into a target response d.sub.k =a.sub.k +a.sub.k-1 or d.sub.k =a.sub.k -a.sub.k-2, and data is detected using a Viterbi decoder. The PRML method shows excellent detecting capacity in a channel having n=1.
RLL codes with "d" greater than zero are not necessary in PRML channels. Since the compensation for the ISI is inherent in the maximum likelihood (ML) detector, there is no need to reduce the interference by coding with a d condition.
Thus, the rate 8/9(0,3) coding and rate 8/9(0,4/4) coding methods are employed in the PRML method utilizing the interference between signals to improve performance with holding high recording density and more timing information.
Also, since the rate 8/9(0,3) coding and the rate 8/9(0,4/4) coding methods have a high code rate, they provide good effect to an equalizer with respect to a given partial response class compared to the rate 1/2(2,7) coding or rate 2/3(1,7) coding method.
If the data sequence of an input signal is divided into an even-bit subsequence and an odd-bit subsequence, ML detection is independently applied to each subsequence. A constraint on the number of successive nominally zero samples in each subsequence adequately limits the detector delay and limits the hardware size. The maximum number of continuous "0"s between "1"s required for each subsequence is called "k1". The condition of k1 required for each subsequence is to reduce a path memory for the ML detector. The RLL(0,k/k1) modulation code satisfying the above condition is the rate 8/9(0,4/4) modulation code.